Thermoplastic resin films, particularly biaxially stretched polyester films, have excellent properties such as mechanical properties, electrical properties, dimensional stability, transparency and chemical resistance; therefore, they have been widely used as substrate films in many applications, including magnetic recording materials and packaging materials. Further, depending on the product constitution, a novel functional layer is often arranged on the surfaces of these thermoplastic resin films. In the polarizer protective film application, as one example of such a functional layer, it has been examined to arrange a biaxially stretched polyester film, which is a thermoplastic resin film, on both sides of a polarizing PVA layer as a protective layer. Further, in another example, as a solar cell backsheet, EVA is arranged on a thermoplastic resin film to form a solar cell.
A polymer multilayer laminated film in which resins having different properties are alternately laminated is capable of exhibiting unique physical properties because of its laminated structure. For example, in Patent Document 1, a laminated film suitably used for glass protection, in which film the tearing resistance is improved by alternately laminating resins showing different mechanical properties, is proposed. Further, in Patent Documents 2 and 3, for example, a heat ray-shielding film which was enabled to selectively reflect light of specific wavelengths, particularly near-infrared radiations, by alternately laminating resins having different refractive indices, and a cold mirror which uniformly reflects visible light are proposed. For these films, utilizing their properties, in addition to those applications where the films are laminated with glass via an adhesive, various other applications have been examined, including laminated glasses prepared by inserting these films between two glass sheets via PVB or EVA and films for solar cell backsheets in which these films are laminated with EVA.
Furthermore, in recent years, in the fields of flat panel displays and touch panels, there is an increasing demand for various optical films such as polarizer protective films and transparent electroconductive films. Particularly, in the polarizer protective film application, for the purpose of cost reduction, it has been actively examined to replace conventional TAC (triacetyl cellulose) films with biaxially-stretched polyester films. However, those biaxially-stretched polyester films that have been examined in the past show higher retardation than TAC films due to the orientation generated in the polymers during stretching; therefore, there is a problem that, when these polyester films are assembled in liquid crystal displays, the retardation of the films causes the generation of interferences color and the quality of the displayed images is thereby deteriorated. In order to solve this problem, retardation-controlling methods have been proposed; however, the level of retardation achieved by these methods is still not satisfactory (for example, Patent Document 4). Alternatively, since retardation is proportional to the film thickness, retardation can also be suppressed by reducing the film thickness in the order of several micrometers; however, an excessive reduction in the film thickness deteriorates the ease of handling, and this approach is thus not practical in the polarizer protective film application. For this problem as well, the use of a polymer multilayer laminated film can potentially control the retardation and, by sandwiching a PVA layer serving as a polarizer with polymer multilayer laminated films having controlled retardation, a high-quality polarizing plate can be obtained.
Still, when other functional layer is arranged as in the above-described applications, the adhesiveness between a thermoplastic resin film and the functional layer presents a problem. For example, those biaxially-stretched polyester films that have been examined in the past do not show sufficient adhesiveness to PVA and EVA and have a problem in that the adhesiveness is deteriorated particularly in high-temperature, high-humidity tests.
In order to solve this problem, for example, with respect to the application for polarizer protection, there have been proposed a method of imparting a polyester film with processability and resistance to high humidity by laminating thereon a resin layer having a low glass transition point (patent Document 5); a method of laminating a resin layer composed of a hydrophilic group-containing copolymer resin on a polyester film (Patent Document 6); a method of incorporating a water-soluble polymer such as a PVA into a resin layer laminated on a polyester film and thereby allowing the resin layer to have a surface energy similar to that of the layer with which the resin layer is adhered (Patent Document 7); and a method of laminating a hydrophilic resin layer on a polyester film by an in-line coating method that applies the resin layer in the production process of the polyester film (Patent Document 8).
However, in the method of Patent Document 5, since a resin layer having a low glass transition point is laminated on the surface of a polyester film, modification of the resin layer at a temperature of the glass transition point or higher may cause whitening of the resin layer and deteriorate its adhesiveness to hydrophilic materials. In addition, when the polyester film is stored in a rolled state, blocking may occur in the rolled polyester film, making the polyester film unsuitable for practical use. Such a method of the Patent Document 6 in which a resin layer composed of a hydrophilic group-containing copolymer resin is laminated on a polyester film is capable of improving the adhesiveness to a certain extent by way of hydrogen bonds formed between the hydrophilic group contained in the resin layer and PVA; however, the improvement is not sufficient and the adhesiveness in a high-temperature and high-humidity environment is not satisfactory. Moreover, in such a method of Patent Document 7 in which a water-soluble polymer is incorporated into a resin layer laminated on a polyester film, the adhesiveness is improved by allowing the resin layer and PVA to have similar surface free energy; therefore, there is a problem with respect to the general adhesiveness in that, for example, even if the adhesiveness to a PVA having a certain saponification degree is improved, the adhesiveness to a PVA having a different saponification degree is deteriorated. In addition, since the resin layer is swollen in a high-temperature and high-humidity environment, the resulting film has poor moist heat-resistant adhesiveness. Such a method of Patent Document 8 where a hydrophilic resin layer is laminated on a polyester film by an in-line coating method that applies the resin layer in the production process of the polyester film also has the same problem as the method of Patent Document 7.